1. Field of Invention
This invention relates to reactive distillation, specifically to improvements in reaction selectivity and conversion, resulting in reduced catalyst volume and capital costs requirement.
2. Discussion of Prior Art
Combining chemical reaction and distillation in one unit operation has found wider acceptance despite intrinsic difficulty in pressure drop across reaction zone. U.S. Pat. No. 4,242,530 for example, taught the method of bundling catalysts in steel wire mesh to form bales, which are then placed inside a distillation column as reactive packing materials. These bales and other currently available reactive distillation packing structural arrangements are all designed to provide free paths for liquid/vapor counter current flow within the reaction zone.
Two major drawbacks come with all existing arrangements:
1) Because of the free paths, both reactants and products can move freely into the reaction catalyst area or remain in the vapor/liquid main stream bulk area, resulting in higher residence time and lower reaction selectivity since desirable product(s) has more time to be converted to undesirable by-products.
2) Part of the product(s) leaving the reaction zone re-enters the reaction zone; either via vapor entering from the stripping zone below the reaction zone; or via liquid entering from the rectification zone above the reaction zone. Such undesirable product movements cannot be prevented because composition profile of any component, including reaction product(s), is continuous and approaches thermodynamic equilibrium within the column. Special catalyst arrangements and internals such as structured packing would further increase capital and maintenance costs, reducing competitiveness.
Accordingly, several objects and advantages of my invention are: This invention focuses on discouraging product re-entry into the reaction zone by positively separating the products from the reactants as soon as the product leaves the reaction zone. This allows desired reaction to proceed with higher degree of selectivity. The following two factors enable this enhancement: Firstly, the rate of products re-entering the reaction zone can be manipulated and optimized, discouraging secondary reactions, generating less by-products. Secondly, the reduced rate of products re-entering the reaction zone lowers product concentration throughout the reaction zone. Equilibrium reaction is encouraged to proceed forward, converting reactants to desirable products faster. Fewer side reactions and faster desirable reaction rate would require less reactor bed volume for a targeted conversion. With shortened residence time requirement, benefits like much shorter bed height amongst others will result. The above revelations open up yet another dimension in the field of reactive distillation.
Reactive distillation can begin accommodating chemical reactions that have previously been precluded due to multiple competing side reactions. Avoiding those side reactions no longer means purposely lowering reaction temperature and suffering low reaction rate. Further objects and advantages of my invention will become apparent from a consideration of drawings and ensuing description.